1. Field of the Invention
The present invention relates to a process for the removal of an oil contained in an aqueous solution of a detergent from the aqueous detergent solution.
2. Description of the Related Art
Foreign matter such as scraps of raw materials adheres on the surfaces of molded or formed products such as machine parts together with an oil. Therefore, such a product has heretofore been immersed into an organic solvent contained in a cleaning tank to dissolve the oil in the organic solvent, thereby cleaning up the foreign matter. Further, the thus-cleaned product has been rinsed with an additional organic solvent and then dried to provide an end product. A chlorofluorocarbon-type solvent has been commonly used as the organic solvent. In recent years, however, the disruption of the ozonosphere around the earth due to chlorofluorocarbon-type solvents has been pointed out. Accordingly, there is a tendency to prohibit the use of such chlorofluorocarbon-type solvents. As a cleaning process substitutable for the process making use of the chlorofluorocarbon-type solvent, there has therefore been investigated a process making use of water as a cleaning fluid.
An example of a cleaning process for cleaning a molded or formed product making use of water as the cleaning fluid includes an ultrasonic cleaning method. According to this method, the molded or formed product is immersed in washing water contained in a cleaning tank equipped with an ultrasonic oscillator on its bottom. An ultrasonic wave is then applied to the washing water through the ultrasonic oscillator to generate cavitation in the washing water, whereby the oil adhered to the surface of the molded or formed product is removed by a shock wave produced at the time the cavitation is broken down.
In the ultrasonic cleaning method, an aqueous solution of a detergent such as a surface active agent is often used as the washing water in order to better enhance the cleaning effect. In the ultrasonic cleaning method, however, the oil adhered to the molded or formed product is cut up into minute droplets having a droplet size of about several microns to several tens of microns when removed from the molded or formed product. The oil is hence easily emulsified by the detergent. The oil emulsified as described above involves a problem that it becomes hard to float on the surface of the aqueous solution, and is difficult to remove from the aqueous solution. Therefore, the amount of oil emulsified in the aqueous detergent solution increases as the aqueous detergent solution is used repeatedly, leading to loss of the detergency of the detergent in a short period of time. Since most of the cost of the ultrasonic cleaning method due to use of the detergent, it is desired that the detergency of the detergent be kept over a long period of time as much as possible.
In order to keep the detergency of the detergent over a long period of time, it has therefore been attempted to remove the oil floating on the surface of the aqueous detergent solution by an oil skimmer. However, only a part of the oil, which is relatively great in droplet diameter and easy to float, can be removed by the oil skimmer because most of the oil contained in the aqueous solution does not float as described above. Accordingly, the method making use of the oil skimmer can not be said to be sufficient for removal of the oil in the aqueous solution.
It has also been attempted to remove the oil in the aqueous solution by means of an oil-separating membrane for separating the aqueous detergent solution and the oil from each other. However, since the oil-separating membrane serves to cause small molecules such as water and the detergent component to selectively pass therethrough, thereby separating the aqueous detergent solution and the oil from each other, the separation of the oil which is cut up into the minute droplets as described above by filtration requires use of an oil-separating membrane with high precision. However, such an oil-separating membrane with high precision involves the following disadvantages. Namely, it tends to be clogged with the oil, though the degree of clogging varies according to the amount of oil to be filtered out. A further washing step for removing such clogging from the membrane takes a long time, and must also use a detergent, resulting in increase in the cost of the ultrasonic cleaning.